The present invention relates, in general, to optics systems (including communication) and elements and, in particular, to optical transceiver with alignment between transmitter and receiver.
Technological advances in semiconductor lasers, photo detectors, and semiconductor sensor arrays have led to laser communication systems being a viable alternative to conventional radio frequency (RF) systems. Free-space laser communication systems are expected to be a significant component of future communications, including terrestrial, airborne, and especially satellite applications.
The short wavelengths of the optical/infrared spectrum allow a transmission to be focused into a narrow beam. A narrow beam allows a great transmission distance. However, accurate pointing of a laser beam between communication terminals requires a spatial acquisition and tracking system. With the present usage of laser communication systems being low as compared to other types of communication systems, the present procedure of assuming that any laser beam received is the intended laser beam is reasonable. Therefore, present manufacturers of laser communication equipment have not seen a need to make any further improvements. Future laser communication usage is expected to be much greater than present usage, and applications may necessitate multiple co-located terminals. As a result, the present procedure could be unreliable and potentially disastrous.
Prior art methods of acquiring a laser beam include fixed mechanical pointing, transmit beam search, retro-reflector search, and use of a dedicated continuous beacon. Each prior art method fails to authenticate the received laser beam or beacon before applying pointing adjustments.
In the fixed pointing method, laser transmitters and receivers are rigidly mounted and manually aimed. Loss of communication in the fixed-pointing method often requires manually adjusting the spatial orientation of the transmitter and/or the receiver.
In the transmit beam search method, automated search and alignment methods are employed to acquire the communication laser beam over an angular search area and a tradeoff exists between search area and search time. The search process is more complex and lengthy when complementary receivers are searching simultaneously for non-stationary beams. Pointing of laser beams at incorrect (unauthenticated) targets will delay or prevent proper alignment.
In the retro-reflector search method, a passive reflector is used to reflect the transmitted laser beam that scans the search area until the reflection is detected. Reflected laser beams from multiple simple reflectors are indistinguishable and proper pointing is again delayed or prevented.
In the dedicated beacon method, a laser beacon is dedicated to the problem of aligning the transmitter and receiver. A laser beacon normally has a larger beam width than a communication laser beam and, therefore, requires fewer search steps, and less time, to align the receiver to the transmitter. Prior art methods employ continuous (unmodulated) beacons. Pointing of laser beams at incorrect (unauthenticated) targets will again delay or prevent proper alignment.
Several pointing alignment methods have been proposed. U.S. Pat. No. 4,401,886, entitled xe2x80x9cELECTROMAGNETIC BEAM ACQUISITION AND TRACKING SYSTEM,xe2x80x9d discloses a device for scanning a laser beam over a field of view and use of a retro-reflector to return the beam to the transmitter that employs a photo multiplier detector. Dithering the beam around a central axis generates the pointing error employed in the steering process. The retro-reflector approach provides a useful beacon for short communication distances. However, reliable pointing demands that only the intended retro-reflector be in the field of view. Any source of reflection other than the reflector would interfere with operation. However, U.S. Pat. No. 4,401,886 does not disclose the present invention. U.S. Pat. No. 4,401,886 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 4,834,531, entitled xe2x80x9cDEAD RECKONING OPTOELECTRONIC INTELLIGENT DOCKING SYSTEM,xe2x80x9d discloses a device that uses independently triggered laser sources for illuminating and isolating a target. U. S. Pat. No. 4,834,531 does not disclose the present invention. U.S. Pat. No. 4,834,531 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 4,867,560, entitled xe2x80x9cOPTICAL ALIGNMENT SYSTEM,xe2x80x9d discloses a device for aligning two transceivers using two beams, where adjustments in alignment are made based on the difference in magnitude between respective quarters of the two beams from a reference value. However, U.S. Pat. No. 4,867,560 does not disclose the present invention. U.S. Pat. No. 4,867,560 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,062,150, entitled xe2x80x9cFIBER-BASED FREE-SPACE OPTICAL SYSTEM,xe2x80x9d discloses a device for coupling received and transmitted light over a physically nutating optical fiber. Modulation of the received signal caused by the nutation is used to make pointing adjustments of a steering mirror to align the transmit/receive beam. However, U.S. Pat. No. 5,062,150 does not disclose the present invention. U.S. Pat. No. 5,062,150 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,119,225, entitled xe2x80x9cMULTIPLE ACCESS COMMUNICATION SYSTEM,xe2x80x9d discloses a device for and a method of locating multiple terminals using a single narrow scanning beacon. However, U.S. Pat. No. 5,119,225 does not disclose the present invention. U.S. Pat. No. 5,119,225 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,142,400, entitled xe2x80x9cMETHOD AND APPARATUS FOR AUTOMATIC ACQUISITION AND ALIGNMENT OF AN OPTICAL BEAM COMMUNICATION LINK,xe2x80x9d discloses a device for and method of establishing a communication link using a search mode, a stare mode, and retro-reflectors. However, U.S. Pat. No. 5,142,400 does not disclose the present invention. U.S. Pat. No. 5,142,400 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,390,040, entitled xe2x80x9cOPTICAL TRANSCEIVER FOR FREE-SPACE COMMUNICATION LINKS,xe2x80x9d discloses a device for acquiring a communication signal using the step-stare approach. However, U.S. Pat. No. 5,390,040 does not disclose the present invention. U.S. Pat. No. 5,390,040 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,465,170, entitled xe2x80x9cALIGNMENT ADJUSTING SYSTEM FOR USE IN OPTICAL SYSTEM OF OPTICAL TRANSCEIVER,xe2x80x9d discloses a device for and method of for overcoming misalignment of optical systems due to thermal expansion. However, U.S. Pat. No. 5,465,170 does not disclose the present invention. U.S. Pat. No. 5,465,170 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,475,520 entitled xe2x80x9cSATELLITE COMMUNICATIONS SYSTEMxe2x80x9d discloses a device for communicating via an optical beam by a first transceiver transmitting a number of signals, each uniquely modulated, a second transceiver returning the signals it receives, and adjusting the transmission area to insure reception by the second transceiver of all the signals transmitted by the first transceiver. However, U.S. Pat. No. 5,475,520 does not disclose the present invention. U.S. Pat. No. 5,475,520 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,592,320, entitled xe2x80x9cSATELLITE COMMUNICATIONS SYSTEMxe2x80x9d discloses a device for communicating via an optical beam by a first transceiver scanning a first beam, a second transceiver scanning a second beam modulated by the time that the first beam was received, the first transceiver scanning modulating the first beam by the time that the second beam was received, and adjusting the scans of both transceivers to be in alignment. However, U.S. Pat. No. 5,592,320 does not disclose the method of the present invention. U.S. Pat. No. 5,592,320 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,594,580, entitled xe2x80x9cOPTICAL SPACE COMMUNICATION APPARATUS,xe2x80x9d discloses a device for and method of propagating a beam of light through free space to effect communication. However, U.S. Pat. No. 5,594,580 does not disclose the method of the present invention. U.S. Pat. No. 5,594,580 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,610,750, entitled xe2x80x9cOPTICAL COMMUNICATIONS TERMINAL,xe2x80x9d discloses a device for inter-satellite communication that includes several telescope modules where each module is operated by a laser, where each beam from the laser may have a data signal modulated thereon, and where one of the modules is used for acquisition and tracking. However, U.S. Pat. No. 5,610,750 does not disclose the method of the present invention. U.S. Pat. No. 5,610,750 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,684,614, entitled xe2x80x9cOPTICAL SPACE COMMUNICATION APPARATUS,xe2x80x9d discloses a device for and method of correcting the angle of light transmitted or received by multiplexing a pilot signal, a transmission signal, filtering the same at the receiver to determine the presence of the pilot beam, and correcting the angle of light transmitted or received. However, U.S. Pat. No. 5,684,614 does not disclose the present invention and is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,710,652, entitled xe2x80x9cLASER COMMUNICATION TRANSCEIVER AND SYSTEMxe2x80x9d discloses a device for communicating via a laser where a beacon beam is recognized and accepted if the frequency of the beacon beam is as expected. However, U.S. Pat. No. 5,710,652 does not disclose the method of the present invention. U.S. Pat. No. 5,710,652 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,867,294, entitled xe2x80x9cOPTICAL SPACE COMMUNICATION APPARATUS,xe2x80x9d discloses a device for performing optical space communication while correcting alignment of an optical axis using an optical element rockable about at least two axes and voice coil motors. However, U.S. Pat. No. 5,867,294 does not disclose the method of the present invention. U.S. Pat. No. 5,867,294 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,923,452, entitled xe2x80x9cLASER SATELLITE COMMUNICATION SYSTEMxe2x80x9d discloses a device for communicating via a laser and employs separate beacons for acquisition and pointing. However, U.S. Pat. No. 5,923,452 does not disclose the method of the present invention. U.S. Pat. No. 5,923,452 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,953,146, entitled xe2x80x9cMETHOD AND APPARATUS FOR TRACKING ALIGNMENT IN WIRELESS OPTICAL COMMUNICATIONS,xe2x80x9d discloses a device for and method of optical communication using an array sensor to recover data that is then evaluated using the principles of geometric invariance to determine the accuracy of alignment. However, U.S. Pat. No. 5,953,146 does not disclose the method of the present invention. U.S. Pat. No. 5,953,146 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,973,310, entitled xe2x80x9cOPTICAL ACQUISITION AND TRACKING SYSTEMxe2x80x9d discloses a method of continuously monitoring the presence and location of an optical signal. However, U.S. Pat. No. 5,973,310 does not disclose the method of the present invention. U.S. Pat. No. 5,973,310 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,991,062, entitled xe2x80x9cMETHOD AND DEVICE FOR OBTAINING ERROR SIGNALS FOR SPATIAL BEAM REGULATION OF AN OPTICAL COHERENT RECEIVERxe2x80x9d discloses a device for and method of aligning two light waves in order to superimpose one on the other. However, U.S. Pat. No. 5,991,062 does not disclose the method of the present invention. U.S. Pat. No. 5,991,062 is hereby incorporated by reference into the specification of the present invention.
It is an object of the present invention to identify (i.e., authenticate) a particular beacon within an angular search area by detecting a beacon intensity-modulated by a specific user-definable bit pattern. The identity of a particular beacon, after being located, can be continuously reconfirmed for tracking purposes.
The present invention is a method of authenticating a beacon.
The first step of the method is establishing a first threshold and a second threshold.
The second step of the method is identifying a current field of view in which a search for a beacon will be conducted, where the current field of view includes a user-definable number of elements.
The third step of the method is initializing each entry in a first array.
The fourth step of the method is recording a frame of signal intensity value for the elements of the current field of view and storing the same in corresponding entries in a second array.
The fifth step of the method is digitizing the signal intensity values recorded in the fourth step using the second threshold.
The sixth step of the method is modifying the values in the first array according to the digitized signal intensity values of the fifth step and a user-definable modulation bit pattern.
If the desired number of frames has not been processed at the current field of view then the seventh step is returning to the fourth step to acquire another frame and continue from the fourth step to process the new frame. However, if the desired number of frames has been acquired and processed then proceed to the next conditional step where the condition is satisfied.
If the first array contains a value that exceeds the first threshold and additional processing is desired then the eighth step is returning to the fourth step to acquire another frame at the current viewpoint and continuing from there to process the new frame.
If the first array contains a value that exceeds the first threshold and the desired number of frames have been acquired and processed, then the ninth step of the method is declaring the beacon at the element in the current field of view that corresponds to the highest value in the first array to be authenticated and stopping.
If none of the values in the first array exceeds the first threshold then the tenth step of the method is setting the current viewpoint to another area in the search area that has not been processed and returning to the third step for additional processing.
If each field of view in a search area is processed and no beacon is found and further processing is desired then the eleventh, and last, step of the method is adjusting (e.g., advancing or delaying) the user-definable bit pattern x bits and using the adjusted pattern as the user-definable bit pattern for processing purposes, ignoring the results of previous steps, and returning to the first step for further processing.